Generally speaking, this invention relates to slide or film projectors having an arc lamp light source, and more particularly to dissolver means for controlling the output light intensity of such projectors.
At the present time, it is known that a plurality of slide projectors may be used to produce an integrated slide or other film show on a single projection screen or on a plurality of screens. In the simplest example, two projectors may be directed to the same screen and may be timed to present slides alternately. Thus, when the first projector is on, its lamp is lighted and the image on a slide is projected onto the screen. When the time arrives for the second slide, the first projector is turned off, i.e. its lamp intensity is either greatly reduced or the light path blocked and the second is turned on, i.e. its lamp intensity is greatly increased or the light path is unblocked. A more complicated application is presented by the complex multiprojector and multi-media shows now being utilized in trade presentations and for entertainment. In such complex shows, a plurality of different, continuously changing images is projected on numerous screens or screen segments, by way of slide and/or movie film.
It is also known that the transition from one projector to another may either be done by a manually operated switch or by means of a program control system connected to the projectors. In use, these controllers allow the transition from one projector to another to be made gradually so that the light from the first gradually decreases while that from the second gradually increases. This type of control is known as a "dissolve" and the device controlling it is known as a "dissolver" or "dissolver controller". There are a number of such dissolver systems now available commercially and in addition to the control switching of projectors, they often incorporate many other features allowing the control of a variety of other projection and audio devices for the most complicated types of presentations. The more complex multi-media and multi-projector shows can only be controlled by such dissolver systems, many of which are computer-controlled.
A further complicating factor is the fact that incandescent lamp-illuminated projectors are only satisfactory for projection over small area screens. As a practical matter, arc lamp projectors must be utilized in very large halls such as theaters or where outdoor or daylight presentations must be made. Such arc lamp projectors cannot be controlled by direct voltage control of the lamp itself since a reduction in current supply to the lamp results in the extinguishing of the lamp. As a practical matter, the control of light intensity of such lamps can only be achieved through a mechanical light valve in the light path. Such light valves have taken the form of an iris, similar to that used in cameras or a set of shutters or flaps likewise adjustable to permit intensity control adjustment by adjusting the opening to pass more or less light according to the needs of the program. One such system is reflected in U.S. Pat. No. 4,158,491 in which an arc lamp projector has a light valve comprising a motor means linearly responsive to average power, for example, a moving iron galvanometer, and a moving means such as a louvre, positioned between the arc lamp and the film gate. In this system, the dissolver operates by phase control of an alternating current supplied to a current responsive motor device which is, in turn, connected by a set of linkages to a light interrupting system. The rotor of the motor has an angle of rotation linearly related to the average output power.
However, it has been found that this system is relatively complex. More desirable would be a system wherein the light valve orientation, and therefore the light intensity projected, were substantially directly proportional to an applied voltage since existing dissolver controllers produce such output voltages for control of audio and incandescent lamp projectors.
By providing a magnetically coupled clutch between a continuously driven motor and the light valve, by providing a source of variable counter-torque and by applying the control voltage to the clutch to control the magnetic coupling thereof, the desired light intensity control may be provided.